Filamentary cathode and method of making same



Sepi. 25, 1956 A. H. WERSEN 2,764,511

FILAMENTARY CATHODE AND METHOD OF MAKING SAME Filed Aug. 28, 1953 Tungsten wire /0 Simered Tungsten Barium Oxide H INVENTOR ARTHUR H. IVERSEN FILAMENTARY CATHODE AND METHOD OF MAKING SAME Arthur Iversen, Kew Gardens, N. Y., assignor to Sylvama Electric Products, Inc., a corporation of Massachusetts Application August 28, 1953, Serial No. 377,166

' 20 Claims. (Cl. 117-217 This invention relates to a filamentary type cathode. More particularly it relates .to a so-called L-type cathode. In recent years Lemmens, Jansen and Loosjes have developed in Holland a cathode of the planar type which has been referred to in the literature asan L-type cathode.

. reservoir for the barium strontium carbonate mixture which is then capped with a porous Wall of tungsten. The molybdenum member is further designed so as to permit a filamentary heater to be used to heat up the material in the cup member. Cathode bodies of this type are, for the most part, fairly large since there is a spacial distance between the filamentary heating element and the molybdenum cup. These cathodes are therefore also relatively slow in heating up.

It is, accordingly, an object of this invention to develop an L-type cathode which can be made in a very small form.

.'It is a further object of this invention to develop an L-type cathode which can be brought to temperature very rapidly.

It is still another object of this invention to provide a method of making an L-type cathode in very simple and inexpensive manner.

These objects and other advantages incidental thereto can be attained 'bycoating a filamentary wire with an emission material, subsequently coating the emission material with a fine molybdenum or tungsten powder and then sintering the final coating. I

In the drawings, which illustrate an embodiment of this invention, Figure 1 is a cross-sectional view of a filamentary .type L-cathode showing the relative position of its component parts.

In the drawing, illustrates a tungsten wire filament coated with barium oxide 12 which has in turn been coated with afine tungsten powder 14 which has subsequently been sintered.

The method of preparation of this filamentary type cathode is pretty much straight forward. The tungsten wire can be coated with emission material, such as barium and strontium carbonates, by either the electrophoretic or bench coating techniques, as well as by any other methods known to the prior art. There is nothing particularly critical inits method of application. It is merely desirablethat a good coherent coating be formed on the wire 10.

The tungsten powder 14 or molybdenum powder which is to be used to form the sintered porous coating over the emissive material is, however, preferably placed thereon in a very finely divided state.

In fact, the tungsten powder might even be referred to as being ultra-fine. They can, of course, be brought onto the emissive coating 12 in any number of ways. The preferred method is one in which the metallic powder is brought on by-the usual bench coating technique from a nitro-cellulose bath containing acetone as a binder. The degree of fineness of the powder will affect the sintering temperatures which can be used for this purpose. When the powder is in a very fine state a radically lower sintering temperature can be used.

During the actual sintering, the emission coating will also break down and thus give a product in which the porous sintered outer coating will contain impregnated therein some of the emissive material, with a layer of the emissive material directly in back of it to act as a reservoir.

In view of the fact that the tungsten wire is in direct contact with the emissive material, it can readily be seen that the resulting cathode structure is of the quick heating type. This is true since the heat can be transmitted directly by conduction and need not depend upon radiation.

When the powdered tungsten or molybdenum powders are sintered, a porous mechanically tight jacket is formed around the emission material. The sintering should be carried out at those temperatures and conditions that will lead it to a sintered mass which is porous, but strong enough not to crumble.

The finished L-tjpe cathode is composed of three parts; a tungsten 'filamentlll', an inner core of barium oxide 12 and an outer sleeve of mechanically tight porous tungsten or molybdenum 14. This cathode is made by electrophoretically coating a tungsten filament wire with a barium carbonate emission material by a standard cataphoretic method in which the barium carbonate is suspended in a solution made up, for example, of 10.4 grams of nitro-cellulose (900-1300 sec.) dissolved in a liter of amyl acetate.

The resulting material is a standard filamentary coated cathode. This coated wire is then bench coated with a fine tungsten powder. The suspension which is used for this purpose is similar to that used for the suspension of the barium carbonate, namely a nitro-cellulose lacquer suspension formed by dissolving 10.4 grams of nitrocellulose in a liter of amyl acetate. The tungsten powder suspended in this solution is one which has a very fine particle size. Suflicient tungsten powder is added to make the suspension slightly thick. After the tungsten powder has been deposited on the carbonate coating the entire structure is brought up to the sintering temperature of the metal powder to produce a reasonably strong mechanically tight sleeve. This temperature is usually in the neighborhood of about 1200- C. For best results at this temperature the particle size of tungsten, if tungsten is'to be used as the outer metal coating, is about $4 of a micron; whereas if molybdenum is used as the outer coating, the particle size is preferably in the neighborhood of .5 micron. In order to get the best results the thickness of the metal powder coating obtained as a result of the bench coating thickness should be about 1 to 2 mils.

When filamentary cathodes of this type are tested it has been observed that emission comes at about 970 C. and then follows a normal curve. It has been further noted that in those cases in which tubes gassed at too high a voltage and flashed over, the emission will build up again upon reducing the voltage to zero and raising it again. This phenomenon is characteristically peculiar to L-type cathodes.

While the above-described manner of preparing the filamentary cathode is preferred, it has been found possible to mix the metal powder and the emission material The coating is carried out under such con-- ditions that a coating thickness of about 1 mil is obtained.

and form a single suspension of the two. This double suspension can then be simultaneously applied to the wire core by either a cataphoretic or bench type coating techniquewhich is then-followed by the normal sintering technique. This latter willtend to break down the carbonate to form the desired oxide and a porous tight sleeve of metal. While this technique results in a cathode coating inwhich the barium oxide is dispersed throughout the sintered tungsten or molybdenum, whichever is used as the metallic sheathforming metal, the structural appearance of the two forms is somewhat different since theform shown in thedrawing has a separate and distinct reservoir of emissive material which can filter up through the porous. metal sheath.

While theabove description and drawing submitted herewith discloses a preferred and practical embodiment of the filamentary cathode of this invention it will be understoodthat the specific details ofconstruction and arrangement of parts as shown and described are by way of illustration and are notto be construed 'as limiting the scope of the invention.

What is claimed is:

1! A filamentary cathode suitable foruse. inan electron discharge device consisting essentially of a tungsten filament. coated .with an. emissive material, the composite being, sheathed. within a porous refractory, metal body.

2. A filamentary cathode suitable for use inian electron. discharge-device consisting essentially of. aitungstentfilament coated with. an. emissive material, the composite being sheathed within a porous tungstenmetal sheath. I

3. .A-filamentary cathode suitable for use in an electron discharge device consisting essentially of a tungsten filament-coated with an emissive material, the compositev being sheathed. within a porous molybdenum metal sheath.

4. A filamentary cathode suitable for use in.an electron discharge device consisting. essentially of a tungsten filament coated with barium oxide, the composite being. sheathed within aporous tungsten metal sheath.

5. In the method of making a filamentary cathode the. steps comprising coating a tungsten filament with an emissive material, applying a coating. containing a finely divided refractory metal and sintering the outer metal coating to form a porous metal sheath.

6. Inthe method of making a filamentary cathode the steps; comprising coating. a tungsten filament with barium carbonate, applying acoating containing a finely divided refractory metal; and sintering the outer metal coating to form a porous metal sheath. 1

7. Inithe method of making a filamentary cathode the steps comprising coating a tungsten: filament with barium carbonate, applying a coating; offinely divided tungsten-particles and-sintering the tungsten-particles to form a tight porous sheath.

8*. In themethodofmaking a filamentary cathode the stepsicomprising coating a tungsten filamenbwith barium carbonate, applying a coating of tungsten powder haviIlgxEPflL'tlClfiSiZG of about of a micron and sintering the composite at a temperature of about 1200 C.

9. ln the method of making a filamentary. cathode, the" steps comprising coating a tungsten filament with barium; carbonate, applyinga coating of molybdenumipowder having a particle size of about .5 micron and sinteringzthe composite at a temperature ofabout1200 C.

10. In the method of making a filamentary; cathode, the .steps comprising coating a tungsten wire with a mixture :ofbarium carbonate and a refractory metalpowder.

and heating the coated wire to the sintering temperature of the metal powder.

11. In the method of making a filamentary. cathode,

63,; the steps comprising coating a tungsten wire with a mixture of barium carbonate and tungsten metal powder and heating the coated wire to the sintering temperature of the metal powder.

12. In the method of making a filamentary cathode, the steps comprising coating .a tungsten wire with a mixture of barium carbonate and molybdenum metal powder and lheatingthe, coatedtwire to the sinteringtemperature of the metal powder.

13. A filamentary cathode. suitable for use in. an electron discharge device comprisingajfilament, a coating. of emissive material cohered to said filament, .andia porous refractory body sheathing said coating, said body being formed by sinteringwherebysome of said emissive material of said coatingimpregnates .saidbody, and said coating serves as a reservoir of emissive material for impregnation into said body.

14. A filamentary cathode suitable for use in an electron discharge device comprising a filament, a coating. of emissive material cohered to said filament, and a porousrefractory body of tungsten powder sheathing said coating, said body being formed by sintering whereby someof said emissive materialof said coatingimpregnates' said body.

15. A filamentary cathode suitable for use in an electrondischarge device comprising a filament, a coating ofemissive material cohered to said filament, and a porous refractorybody ofmolybdenum sheathing said. coating, saidibody being formed by sintering whereby some of said 'emissive material of said coating impregnates said body, and saidcoating serving as a reservoir of emissive material for impregnation into said body.

16. A quick-heating cathode for. an electron tube cornprising a filament, a coating of emissive material in direct contact with said filament, a sintered and mechanicallyrigid metallic body' sheathing said coating, said metallic body being. porous and having difiused therein emissive material of said coating.

117: A quick-heating cathode for anelectron tube comprising a tungsten filament, a coating. of emissive material in direct contact with said filament, a sintered-and mechanically-rigid body of tungsten sheathing said coating, said metallic body being porous and having dififused thereinemissive material of said coating.

18. A quick-heating cathode for an. electron-tube comprising a tungsten filament, a coating of emissive material indirect contact with saidfilament, a sinteredand mechanically-rigid body of molybdenum sheathing said coating, saidmetallic body being porous and having difiused therein emissive material of said coating.

=19. A cathode comprising a heatable body,\a coating of emissive material cohered to said heatable body, and a porous refractory body of metal powder sheathing said coating, said porous refractory body being formed by sintering whereby some of said emissive material of said coating-.impregnates said porous refractory body.

20. A cathode comprising a heatable body, a coating of emissive material in direct contact with said heat-able body, and a sintered and mechanically-rigid metallic body sheathing said coating, said metallic body being porous and having diffused therein emissive material. of said coating, said coating serving as a reservoir of emissive material, for further diifusion into said metallic body.

References .Cited in the file of this patent UNITED STATES PATENTS 2,041,802 Wilson et al May 26, .1936 2,142,331 Prescott Jan. 3, 1939 2,297,467 Gorlich Sept. 29, 1942 2,353,635 Aicher July 18, 1944 

5. IN THE METHOD OF MAKING A FILAMENTARY CATHODE THE STEPS COMPRISING COATING A TUNGSTEN FILAMENT WITH AN EMISSIVE MATERIAL APPLYING A COATING CONTAINING A FINELY DIVIDED REFRACTORY METAL AND SINTERING THE OUTER METAL COATING TO FORM A POROUS METAL SHEATH. 